Acoustic restoration: using soundscapes to benchmark and fast-track
rehabilitation of ecological communities
Elizabeth Znidersic* and David M Watson
Address and affiliations: School of Agricultural, Environmental
and Veterinary Sciences
Charles Sturt University
Albury NSW 2640 Australia
*Corresponding author :
eznidersic@csu.edu.au
Word count: 1,998 words (excluding 59 word abstract, 50
references, one table and one figure)
Submitted: 18 November 2021, to be considered for publication
in Ecology Letters as a Viewpoint (initial proposal approved by
senior editor Peter Thrall, 29 September 2021)
Keywords: Ecoacoustics, social attraction, microbial ecology,
engagement, disturbance
Abstract: We introduce a new approach—acoustic
restoration—focusing on the applied utility of soundscapes for
restoration, recognizing the rich ecological and social values they
encapsulate. Broadcasting soundscapes in disturbed areas can accelerate
recolonization of animals and the microbes and propagules they carry;
long duration recordings are also ideal sources of data for benchmarking
restoration initiatives and evocative engagement tools.
Authorship statement: EZ responsible for the original concept
of acoustic restoration, wrote the sections on acoustic lures and
soundscape ecology. DMW responsible for the idea of microbial
augmentation and aquatic applications, wrote majority of the first
draft. EZ and DMW equally responsible for concept of acoustic
benchmarking, both authors contributed to revisions.
Data accessibility statement: No new data are presented in this
manuscript, the long duration acoustic data summarized in Fig. 1 are
derived from the open access Australian Acoustic Observatory,
https://acousticobservatory.org
As catastrophic wildfires, heatwaves and storms increase in frequency
and severity, both terrestrial and aquatic ecosystems face mounting
pressures, pushing relictual populations towards local extinction and
compromising ecosystem function. Current restoration praxis was
originally developed as site-scale secondary interventions—removing
primary stressors and relying on natural regeneration to do the rest.
While this passive approach has made way for active interventions (Table
1), most are too effort-intensive to apply beyond the individual site
scale. Out-of-the-box thinking is urgently needed to develop scalable
and rapidly deployable methods to arrest further declines, complimenting
existing interventions to facilitate recovery.
After a sustained period of innovation and growth, a critical mass of
acoustic ecology research and practice has been reached, evidenced by
large-scale government investment in infrastructure (Roe et al.2021), international data-sharing networks and adoption of standard
operating procedures to maximise comparability (Browning et al.2017). An idea that emerged from our use of sound to survey both species
and communities over the past decade is to reimagine this monitoring
tool as an active restoration approach. We name this new field
“acoustic restoration”, emphasising soundscapes as holistic high
resolution digital depictions of their respective landscapes (Schafer
1977) and recognizing the biological, geophysical and socio-cultural
values they encapsulate (Pijanowski et al. 2011; Parker &
Spennemann 2021).
Here we introduce the idea and develop four elements of this novel
transdisciplinary domain. The first broadens existing use of acoustic
lures to attract single species up to entire assemblages, broadcasting
soundscapes to fast-track recolonization of communities from the top
down. The second element uses increased animal visitation to augment the
rain of seeds, spores, bacteria and fungi to re-inoculate aquatic and
terrestrial communities, rehabilitating ecosystems from the bottom up.
Third, we suggest sound represents an ideal benchmark for restoration,
providing an independent and verifiable means of answering the
question—are we there yet? Finally, we advocate using soundscapes as
evocative engagement tools to remind stakeholders what their river, reef
or rainforest sounded like and create new ways to reconnect with places
they hold dear.
Acoustic lures are an existing tool in the restorationist’s repertoire,
used for various vocal animal groups to elicit a response for detection,
capture or attract individuals to specific locations and encourage
breeding. To accelerate establishment or recolonisation through social
attraction, species-specific acoustic lures have been applied
successfully to frogs (James et al. 2015), seabirds (Arnoldet al. 2011; Herrera-Giraldo et al. 2021), bats (Ruffellet al. 2009), coral reef fish (Gordon et al. 2019) and
whales (Tyack et al. 2011), proving especially effective for
long-lived colonial animals that remember using that location prior to
disturbance (Jones & Kress 2012). Putman and Blumstein (2019) flagged
the potential for using call playback to recruit animals into newly
restored habitats but expanding this approach up to community-scale
interventions has not been considered. As well as animal vocalizations,
sounds emanate from other biological, anthropogenic and geophysical
sources which can act as individual or collective cues for species
(Pijanowski 2011). In addition to indicating where to go (e.g.,settlement response to reef sounds by crab and oyster larvae; Stanleyet al. 2009, Lillis et al. 2013), not go (e.g.,avoidance of anthropogenic sounds by cetaceans and fruit bats; Tyacket al. 2011, Ruffell et al. 2009), or how to get there
(e.g., encouraging frogs to cross railways via wildlife
underpasses; Testud et al. 2020), sounds can alter behaviours
(e.g., rainfall sounds trigger breeding behaviour in frogs; Muñozet al. 2020).
The same technology used to make passive acoustic sensors for collecting
long-duration recordings can readily be repurposed to make autonomous
playback devices, matching the soundtrack to the location and substrate,
and optimising the duty cycle to the target species and acoustic theatre
(e.g., nocturnal playback of flight calls for passage migrant
birds, broadcasting frog choruses after significant rainfall). By
playing segments of an entire soundscape or curated compilations of key
vocal species (‘mix tapes’), concerns over temporary habituation can be
minimized noting that, despite oft-shared anecdotes, direct evidence of
deleterious impacts of call playback is scant (Watson et al.2018). Comparing mix tapes with natural or edited soundscapes,
mechanistic cues used by various animal groups can be identified,
allowing progressively more tailored lures for particular restoration or
remediation contexts. Pairing the use of lures with sensor-based surveys
(passive acoustic recorders, camera traps, even web-cams), large-scale
restoration initiatives can be conducted and monitored in remote and
inhospitable landscapes, control sites restored using conventional
practices providing time-matched counterfactuals to quantify any initial
or medium-term differences. In addition to minimizing demographic and
genetic losses from initial disturbance, fast-tracking recolonization
can prevent encroachment of despotic species that aggressively exclude
subsequent colonists from the original assemblage (Leseberg et
al. 2015).
Regardless of whether visiting animals decide to stay, simply attracting
passing animals to target sites will augment recolonization of the
bacteria, fungi, protists and plankton that perform foundational roles
in food webs. Mycorrhizal fungi can take decades to return after
catastrophic wildfire (Dove & Hart 2017), while the microbial films
that underly energy flux in freshwater systems can take over a century
to recover from industrial pollution (Vrba et al. 2003). The
simplified microbial communities that characterize disturbed systems
diminish their resilience, increasing sensitivity to additional
disturbance events. The idea of using visiting animals to fast-track
restoration has been trialled before, primarily in reforesting
agricultural land where the addition of artificial perches to cleared
areas facilitates dispersal of large-seeded plants by visiting birds
(Wunderle 1997, Athiê and Dias 2016). Although more relevant in
terrestrial systems, the potential for wide-ranging aquatic organisms to
seed microbial recovery has been noted by researchers working on both
marine reefs (e.g., fish accelerating recovery of coral
endosymbionts after bleaching events; Grupsta et al. 2021) and
freshwater wetlands (e.g., the microbiome of fish homogenizing
river bacterial communities; Zha et al. 2020). In addition to fungi and
bacteria, seeds and small animals are transported by birds (Fontenato
2019, González-Varo et al. 2019) and fish (Schofield et
al. 2018, Goulding et al. 1990) effecting long-distance
dispersal across inhospitable intervening areas.
A frequently recognized failing of restoration initiatives is brokering
agreement on the answer to the question: ‘What does success look like?’
(after Prach et al. 2019). Acoustic restoration recasts this
question as “What does success sound like?”. For mining and other
commercial infrastructure development, pre-disturbance recordings from
impacted sites offer a quantifiable benchmark for future restoration
practitioners to work towards. For already disturbed sites, soundscapes
from adjacent areas or ecologically similar reference sites can provide
high resolution data about both species assemblages and structural
characteristics that can be logistically complex to estimate at the
whole-of-system scale. Current ecoacoustics allows a suite of metrics to
be extracted from recordings (Fig. 1), including species richness
(Towsey et al. 2013) and identification of dominant taxa, but
also seasonal dynamics, breeding events, even estimating canopy
complexity by quantifying how sound from storms dissipates (Haskell
2020). Progressive monitoring of restored sites will reveal which
targets are met and which are yet to be attained, prioritising on-ground
actions to optimise recovery. Noting recent advances in estimating
abundances, identifying individuals, detecting reproductive events, mass
flowering and even predation success with current analysis and
visualisation techniques (Browning et al. 2017 and references
therein), burgeoning ecoacoustics research will enable future
practitioners to extract progressively more historic information from
archived recordings, giving restoration practitioners a trove of
pre-disturbance metrics to gauge the functional success of their work.
Finally acoustic restoration offers unparalleled opportunities for deep,
meaningful engagement. Sounds are evocative and every place has its own
soundscape (Schafer 1977, Pijanowski 2011). Farmers remember curlews
calling on moonlit nights when they were children (Robb et al.2012), chorusing cicadas alert rainforest people to heat waves in the
treetops (Feld et al. 2020). The sounds of animals and particular
winds or waves feature strongly in First Nation accounts of places of
cultural significance (Parker & Spennemann 2021). In addition to
rallying communities to restore connections with what places once
sounded like, natural sounds have a range of health benefits (Buxtonet al. 2021), tangible reminders of the value of immersive
outdoor experiences. Natural sounds transcend human language, online and
mobile platforms defining new ways for the environment to project its
own voice into the boardrooms, studios and chambers where critical
decisions are made. More soberingly, as development escalates and entire
biomes make way for production agriculture and aquaculture, archived
soundscape recordings and the whole-of-assemblage permanent records they
represent will remind people what wild places were once like.
Acoustic ecology has surged in popularity as a compliment to existing
ecological techniques, due primarily to the rich resolution and archival
stability of acoustic data. As the ‘hype’ recedes and our
transdisciplinary field matures, we see great benefits of applying
acoustic ecology to the practice of ecological restoration. As well as
cost-effective and nimble (deployable within hours of disturbance
events), acoustic restoration minimises the need for ongoing visits by
teams of people, reducing risks of site disturbance and inadvertent
introduction of invasive species and pathogens. In addition to wildfire,
coral bleaching, blackwater events and catastrophic storms, acoustic
restoration could be used proactively to push range shifts toward
unoccupied but otherwise suitable future habitats to minimize climate
impacts, for both resident and migratory assemblages in insular and
extensive systems. Advances in eDNA sequencing and semi-automated
identification using DNA-barcoding entrain reliable means of quantifying
change in microbial communities—both occurrence and genetic
interchange—with parallel work on seedling emergence, soil and water
properties enabling quantification of changes to plant populations and
ecosystem health. Rather than chasing shifting baselines or arguing
about the unreliability of indicator taxa or space-for-time
substitutions, archived open access soundscapes can guide diverse
stakeholder groups towards a common purpose, defining on-ground work
towards agreed targets representing the true complexity of ecosystems.
To realise these benefits and maximise the utility of acoustic
restoration, we suggest four priority actions. First, we urge empirical
ecologists to collect long duration recordings as part of their
fieldwork. With equipment now readily available, recording soundscapes
and associated metadata should be as routine as taking photographs of
your study area. As large distributed arrays of acoustic sensors are
being established to track environmental change at continental scales
(Roe et al. 2021), investment will be increasingly directed
towards platforms to store, share and visualise these data. Second, we
encourage researchers and practitioners alike to listen to their
systems. The simple observation that “higher quality woodlands rustle
underfoot” (Freudenberger, in litt .) presaged the importance of
productivity, litterfall and litter-dwelling invertebrates in driving
woodland food-webs (Watson 2011). The very act of recording sounds
increases one’s awareness of the surrounding landscape (Feld et
al. 2020) and helps tune one’s understanding of the underlying
variability and constitutive complexity. Third, think beyond species.
While species recognition is increasingly achievable for many animal
groups, using ecoacoustics to quantify species richness is akin to using
satellite photography to identify vegetation types. It’s not the best
tool for the job. False colour spectrograms and other applications of
acoustic indices are readily able to extract a variety of metrics from
recordings, many of which are likely influenced by the same suite of
underlying mechanisms that determine species occurrence and community
composition. Looking past species to these biotic and abiotic gradients
will reveal new variables that ecoacoustics is far better suited to
quantify—the underlying topography sculpting productivity,
seasonality, resilience and energy flux. Finally, collaborate; with
acoustics specialists that can test microphones, calibrate equipment and
ensure metadata are associated and complete; with environmental DNA
specialists that can take a vial of water or bag of soil and tell you
how many species of salamander live in that forest; with microbial
ecologists that can take those samples and quantify how many taxa have
recovered in that site since the last samples were taken. Restoring our
streams and grasslands, our mangroves and estuaries, wetlands and
saltmarshes is a top priority, remediating past damage and responding
rapidly to future disturbance. Grounded in collaboration and facilitated
by digital technology, acoustic restoration compliments existing
on-ground approaches using the unique properties of sound to accelerate,
augment, benchmark and engage.